The growing influence of the Internet on human communications has increased the demand for the availability of Internet-based services. This for instance has led to mobile networks offering Internet services to allow the users of the network Internet access. However, offering Internet access on a mobile network is not without technical complications. The Internet protocol (IP) was designed for use with fixed hosts and does not consider support for user mobility.
Efforts have been made to provide mobile, e.g. wireless, networks that support communication using IP. Such networks are typically referred to as mobile IP networks. Early efforts to support mobility on such networks include solutions that handle host mobility at the network layer, such as Mobile IP, Mobile IPv4 and Mobile IPv6. More recently, solutions for supporting host mobility at the application layer have been proposed. Of these solutions, the Session Initiation Protocol (SIP) appears to be the most promising implementation of such a solution.
The SIP is a simple text-based application-layer protocol that is gaining wide acceptance as the call control protocol for multimedia services in IP networks. The 3rd generation partnership project (3GPP) has already adopted SIP for multimedia call control in next generation networks. SIP can intrinsically handle user mobility, since it uses logical addresses, i.e. SIP Uniform Resource Identifiers (SIP-URI) for identifying SIP users, independent of the device they are currently using.
SIP-based communication protocols may be extended to also cover terminal mobility including service coverage in roaming network domains. This is referred to as Mobile SIP. According to this approach ongoing SIP sessions need to be re-established after handoff to a new subnet router. End-to-end SIP session re-establishment messages are exchanged, containing the new IP address of the mobile host and updated session description for the ongoing session. This enables the correspondent host to send all subsequent data to the new IP address of the mobile host. This may be unacceptable if the session has to comply with Quality of Service (QoS) requirements.
The roaming network domain may comprise of a plurality of subnets. Mobile SIP is not suitable for intra-domain handoffs because it involves end-to-end signaling exchange for IP address update after handoff to a new subnet router within the domain. An alternative to Mobile SIP will be to assign two IP addresses to the mobile host—a local IP address that has validity within the domain that the host is currently roaming and a global IP address that has validity outside the domain. The global IP address will be registered with the home network of the terminal. In this case, the domain router of the roaming network domain can tunnel the packet comprising the global IP address in its header to the subnet associated with the terminal. Such tunneling typically includes wrapping the packet with a wrapper including a header specifying the local IP address such that the registration details stored on the home network of the mobile terminal do not need to be updated every time the mobile terminal switches between subnets in the roaming network domain. The mobile terminal also tunnels outgoing packets using such a wrapper. A drawback of this approach is that it creates additional overhead in terms of packet size. This can be particularly undesirable when the packets themselves are small. For instance, in the IPv4 standard, a minimum of an additional 20 octets will be added to all the packets because of tunneling.